It is known in the prior art to employ multiple-wound three-phase motors driven by a corresponding number of PWM inverters, each inverter corresponding to a winding set on the motor. It is also known to allow functioning inverters to operate the motor following failure of one or more of the inverters. In such a case, each inverter drives the winding set to which it corresponds, and the failed inverters are cut off from the system. It is possible to use a tandem motor configuration so that the functioning inverters can operate the motor without switching. However, tandem configurations are costly and require excessively long shaft length. For a double wound tandem configuration, the shaft is twice as long as a single wound motor.
It has been shown in Japanese Patent Application No. SHO 62 [1987]-64957 two separate inverters are coupled with a phase-to-phase reactor to drive a single motor; if one inverter unit fails, it is disconnected and the motor is operated at lower speed and torque with the functioning inverter. In Hokari et al, "Drive System of Ultra High Speed Elevator", 1994 National Conference of the Electric Association, No. 551, pp. 5-91 to 5-92, each of two independent windings on a double wound motor are driven independently with a corresponding inverter; when one inverter fails, it is disconnected, and the other winding set is driven with the functioning inverter. Japanese Kokoku patent No. HEI 7[1995]-002037 causes the current command to be reduced by one-half to operate a three-phase double wound motor following the failure of one inverter. When one functioning inverter is used to drive one winding set of a double wound motor, following failure of the other inverter, the excitation current command, I.sub.0 *, and torque current command, I.sub.T *, within the motor become one-half of what they would be with both inverters driving the motor, so that motor torque is reduced to one-quarter of normal torque.